1. Field of the Invention
The present invention relates to a receiver, and more particularly to a receiver incorporating a mixer that converts a received radio-frequency signal into a signal having a predetermined frequency.
2. Description of Related Art
In general, a receiver employing a radio-frequency wireless system such as satellite broadcast, satellite communication, or wireless telephony receives a radio-frequency signal in the GHz band to reproduce an original signal therefrom. FIG. 8 is a block circuit diagram of a typical front-end portion of a radio-frequency circuit used in such a receiver.
A radio-frequency signal received via an antenna 120 of a receiver is fed, via an input terminal 101 of a front-end portion 110, to a low-noise amplifier 102 that amplifies the radio-frequency signal, which is in the GHz band. The amplified radio-frequency signal is then fed to one input terminal of a mixer 106 provided in the succeeding stage. To the other input terminal of this mixer 106 is fed, from a local oscillator 104, a local oscillation signal having a frequency different from that of the received radio-frequency signal. As a result, an intermediate-frequency signal having a frequency equal to the difference between the frequency of the received radio-frequency signal and that of the local oscillation signal is outputted from the mixer 106 to an output terminal 105. This intermediate-frequency signal is then amplified by an intermediate-frequency amplifier 130 provided in the succeeding stage, and is then subjected to predetermined processing for signal reproduction.
Incorporating the front-end portion 110 configured as described above, the receiver has the following problem. The input/output linearity characteristic of the low-noise amplifier 102, in particular that of the mixer 106, greatly affects the distortion that the signal suffers in the receiver. That is, with a poor input/output linearity characteristic, when a signal with a high electric-field strength is received, the signal obtained by receiving that signal suffers distortion.
FIG. 9 is a diagram schematically showing the input/output characteristic of a mixer that is commonly used in a radio-frequency circuit to produce an intermediate-frequency signal. A receiver is required to be capable of receiving a signal of which the electric-field strength varies in a wide range. Thus, in the receiver, a signal of which the electric-field strength varies in a wide range is amplified by a low-noise amplifier having a predetermined gain, and is then fed to a mixer. In FIG. 9, the horizontal axis represents the signal strength of the signal that is fed to the mixer, which signal strength is referred to as the input signal strength. The input signal strength is proportional to the electric-field strength of the received radio-frequency signal. The vertical axis, on the other hand, represents the signal strength of the intermediate-frequency signal that is outputted from the mixer, which signal strength is referred to as the output signal strength.
In FIG. 9, the range indicated by C is the range of the input signal strength of the mixer, i.e., the range in which the input signal strength is proportional to the received electric-field strength. The input/output linearity characteristic curve P11 indicates an ideal linear relationship between the input and output signal strengths. By contrast, the input/output linearity characteristic curve P12 indicates the input/output linearity characteristic actually observed with a mixer 106 commonly used, for example, in a conventional front-end portion 110 as shown in FIG. 8. A mixer should ideally exhibit a linear relationship over the entire range C as indicated by the input/output linearity characteristic curve P11. In reality, however, a commonly used mixer, while exhibiting a close-to-linear characteristic in an ordinary-input-signal-strength range A, i.e., the range where the input signal strength is ordinary, exhibits a non-linear characteristic in a high-input-signal-strength range B, i.e., the range where the electric-field strength, and thus the input signal strength, is high.
For this reason, in a front-end portion incorporating a single-gain low-noise amplifier, when a signal having a high electric-field strength is received, in the stage succeeding the low-noise amplifier, the intermediate-frequency signal suffers distortion mainly attributable to the input/output linear characteristic of the mixer, with the result that the data carried by the signal is damaged. One solution to this problem is to design the front-end portion, in particular the mixer, to exhibit an ideal linearity characteristic over the entire range C mentioned above. Undesirably, however, designing the mixer that way results, as an inevitable consequence attributable to the characteristics of a transistor, increased current consumption by the transistor. In particular in portable wireless telephones and portable wireless communication apparatuses such as personal digital assistants incorporating wireless communication circuits, minimizing the current consumption is one of the important challenges.
For this reason, reduction of signal distortion is often attempted by lowering the gain of a low-noise amplifier when a signal having a high electric-field strength is fed to the low-noise amplifier. This is exemplified by the technique disclosed in Japanese Patent Application Laid-Open No. H7-30445. According to this technique, the electric-field strength of a received signal is detected, and, if the detected electric-field strength is higher than a predetermined value, the gain of a low-noise amplifier is lowered.
Varying the gain of a low-noise amplifier, however, requires the use of a variable-gain low-noise amplifier. A variable-gain low-noise amplifier, to permit the gain thereof to be varied, requires many extra circuits, and is thus more likely to generate noise than a fixed-gain low-noise amplifier. Moreover, even a variable-gain low-noise amplifier is not free from conventionally experienced problem when fed with a signal having an electric-field strength higher than it can cope with.